A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
Lithography is widely recognized as one of the key steps in the manufacture of ICs and other devices and/or structures. However, as the dimensions of features made using lithography become smaller, lithography is becoming a more critical factor for enabling miniature IC or other devices and/or structures to be manufactured.
A lithographic apparatus typically includes an illumination system configured to condition a radiation beam; a support structure constructed to hold a patterning device, such as a reticle or mask, the patterning device being capable of imparting the radiation beam with a pattern in its cross-section to form a patterned radiation beam; a substrate table constructed to hold a substrate; and a projection system configured to project the patterned radiation beam onto a target portion of the substrate.
A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1):
                    CD        =                              k            1                    *                      λ                          NA              PS                                                          (        1        )            where λ is the wavelength of the radiation used, NAPS is the numerical aperture of the projection system used to print the pattern, k1 is a process dependent adjustment factor, also called the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed feature. It follows from equation (1) that reduction of the minimum printable size of features can be obtained in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NAPS, or by decreasing the value of k1.
In order to shorten the exposure wavelength and, thus, reduce the minimum printable size, it has been proposed to use an extreme ultraviolet (EUV) radiation source. EUV radiation sources are configured to output a radiation wavelength of about and/or below 13.5 nm. Thus, EUV radiation sources may constitute a significant step toward achieving small features printing. Such radiation is termed extreme ultraviolet or soft x-ray, and possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or synchrotron radiation from electron storage rings.
Desirably, both the illumination system and the projection system include a plurality of optical elements in order to focus the radiation on the desired locations on the patterning device and the substrate, respectively. Unfortunately, apart from some gases at low density, no materials are known to be transmissive to EUV radiation. Therefore, the lithographic apparatus using EUV radiation does not employ transmissive lenses in its illumination system and in its projection system. Instead, the illumination system and the projection system desirably include mirrors. In addition, the patterning device is desirably a reflective device, e.g. a mirror having a reflective surface provided with a pattern formed by an absorptive material on the reflective surface, for the same reason.
To reflect EUV radiation having a wavelength of about 6.9 nm, multilayer mirrors have been proposed having alternating layers of a metal, such as (amongst other examples) La, U or Th, and B or a B compound, such as B4C or B9C. Such a multilayer mirror reflects the EUV radiation according to Bragg's Law. For good optical performance of the multilayer mirror, it is desirable to have a sharp interface (i.e. transition) between the alternating layers. However, interlayer diffusion can arise, which may reduce this sharpness and thus adversely affect the optical performance (e.g. reflectivity) of the resultant multi-layer mirror.
Interlayer diffusion may arise due to a chemical interaction between materials forming the different, alternating layers. For example, at an interface between B on La, high chemical reactivity may be present between B and La, leading towards the formation of LaB6, and a reduction in sharpness of the interface between the B and La layers. This process also occurs when B is substituted by B4C. In another example, at an La on B (or B4C) interface, high kinetic energy of relatively heavy La atoms arriving at the surface of relatively light B (or B4C) layer atoms results in implantation of the B (B4C) layer with La up to a depth of around 2 nm. Such implantation can result in a reduction in sharpness of the interface between the La and B layers.